The initial response to interruption of continuity of a blood vessel is defined as primary haemostasis. Platelets play a major role in the pathophysiology of primary haemostasis. The clinical importance of platelets first became obvious when thrombocytopenic patients, who later on were diagnosed as having immune mediated thrombocytopenia (ITP), had thrombocytopenic purpura. Platelets participate in haemostasis by sealing vascular injuries and by fostering the process of blood coagulation. Not only the number of the platelets is important (thrombocytopenia for whatever reason) but also their intrinsic function upon activation: change in platelet shape, adhesion, aggregation and secretion are prerequisites for normal haemostasis. Congenital or acquired disorders interfering with one of the platelet functions can lead to mild to even severe bleeding problems.
Prevention and treatment of bleeding in patients with thrombocytopenia or thrombocytopathia is therefore based on platelet transfusion or medication interfering with platelet number and/or function.
Platelets also play a role in the development of arterial thrombosis. Disruption of the endothelial cell lining of the vessels exposes adhesive proteins within the subendothelial matrix, leading to platelet attachment. Thereafter, platelet spreading, as well as platelet secretion occurs. The secretion of the content of platelet granules can stimulate circulating platelets to acquire new adhesive properties. Finally, stimulated platelets interact with each other during platelet aggregation and a platelet rich thrombus is formed, which can compromise the patency of blood vessels. Furthermore activated platelets accelerate the rate of activation of coagulation proteins. Phospholipids on the platelet surface facilitate thrombin generation and fibrin strand formation.
Arterial and venous thrombosis and their complications including ischemic stroke, acute myocardial infarction and venous thromboembolism, represent the major cause of morbidity and mortality in developed countries.
Prevention and treatment of thrombosis are therefore based on administration of antiplatelet drugs, anticoagulants or thrombolytic therapy or combinations thereof.
The pituitary Adenylyl Cyclase Activating Peptide (PACAP 1-38) is a 38-amino acid peptide that was first isolated from ovine hypothalamic extracts on the basis of its ability to stimulate cAMP formation in anterior pituitary cells [Miyata A. et al., (1989) Biochem Biophys Res Commun 164, 567-574; Vaudry D. et al. (2000) Pharm Rev 52, 269-324]. PACAP is a member of the Vasoactive Intestinal Polypeptide (VIP)—glucagon-growth hormone releasing factor-secretin superfamily. Its role in biology is probably crucial, since the sequence of PACAP is highly conserved during the evolution from protochordate to mammals. PACAP is widely expressed and occurs in the central and peripheral nervous system, the urogenital system, the gastrointestinal tract, and in several endocrine glands. PACAP receptors are also widely distributed (Vaudray et al. cited supra). Two classes of PACAP binding sites have been characterized based on their relative affinities for PACAP and VIP: type I binding sites with high affinity for PACAP (Kd=0.5 nM) and much lower affinity for VIP (Kd>500 nM) and type II binding sites, which are widely distributed in various peripheral organs, characterised by similar affinities for PACAP and VIP (Kd=1 nM). Molecular cloning of PACAP receptors has demonstrated the existence of three distinct receptor subtypes that are abundantly spread in many tissues: the PACAP-specific PACAP receptor, coupled to different signal transduction systems, and two PACAP: VIP-indifferent receptors (VPAC1 and VPAC2, also referred to as VIPR1 and VIPR2, respectively), which are primarily coupled to adenylyl cyclase.
The exact biological and pharmacological function of PACAP is presently being investigated in many organs and tissues as in endocrine glands, central nervous system, respiratory system, cardiovascular system and gastrointestinal tract. Although extensive studies have also been performed on its function in the immune system, limited data are available concerning its function on haemostasis. An effect of PACAP on aggregation and metabolism of isolated platelets has been described [Kis et al. (1999) Prostaglandins Other Lipid Mediat. 58:103-112 and Ichiki et al. (1992) Biochem Biophys Res Commun. 187, 1587-1593]. The occurrence of the VPAC1 receptor on platelets which binds both VIP as PACAP has been reported (Park et al. (1996) Blood 87, 4629-4635).